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152.3.3.1 Surface Solar Radiation
Changes in radiative fluxes at the surface can be traced further back in time than the satellite-based TOA fluxes, although only at selected terrestrial locations where long-term records exist. Monitoring of radiative fluxes from land-based stations began on a widespread basis in the mid-20th century, predominantly measuring the downward solar component, also known as global radiation or surface solar radiation (SSR). AR4 reported on the first indications for substantial decadal changes in observational records of SSR. Specifically, a decline of SSR from the beginning of widespread measurements in the 1950s until the mid-1980s has been observed at many land-based sites (popularly known as ‘global dimming’; Stanhill and Cohen, 2001; Liepert, 2002), as well as a partial recovery from the 1980s onward (‘brightening’; Wild et al., 2005) (see the longest available SSR series of Stockholm, Sweden, in Figure 2.13 as an illustrative example). Since AR4, numerous studies have substantiated the findings of significant decadal SSR changes observed both at worldwide distributed terrestrial sites (Dutton et al., 2006; Wild et al., 2008; Gilgen et al., 2009; Ohmura, 2009; Wild, 2009 and references therein) as well as in specific regions. In Europe, Norris and Wild (2007) noted a dimming between 1971 and 1986 of 2.0 to 3.1 W/m–2 per decade and subsequent brightening of 1.1 to 1.4 W/m–2 per decade from 1987 to 2002 in a pan-European time series comprising 75 sites. Similar tendencies were found at sites in northern Europe (Stjern et al., 2009), Estonia (Russak, 2009) and Moscow (Abakumova et al., 2008). Chiacchio and Wild (2010) pointed out that dimming and subsequent brightening in Europe is seen mainly in spring and summer. Brightening in Europe from the 1980s onward was further documented at sites in Switzerland, Germany, France, the Benelux, Greece, Eastern Europe and the Iberian Peninsula (Ruckstuhl et al., 2008; Wild et al., 2009; Zerefos et al., 2009; Sanchez-Lorenzo et al., 2013). Concurrent brightening of 2 to 8 W m–2 per decade was also noted at continental sites in the USA (Long et al., 2009; Riihimaki et al., 2009; Augustine and Dutton, 2013). The general pattern of dimming and consecutive brightening was further found at numerous sites in Japan (Norris and Wild, 2009; Ohmura, 2009; Kudo et al., 2011) and in the SH in New Zealand (Liley, 2009). Analyses of observations from sites in China confirmed strong declines in SSR from the 1960s to 1980s on the order of 2 to 8 W m–2 per decade, which also did not persist in the 1990s (Che et al., 2005; Liang and Xia, 2005; Qian et al., 2006; Shi et al., 2008; Norris and Wild, 2009; Xia, 2010a). On the other hand, persistent dimming since the mid-20th century with no evidence for a trend reversal was noted at sites in India (Wild et al., 2005; Kumari et al., 2007; Kumari and Goswami, 2010; Soni et al., 2012) and in the Canadian Prairie (Cutforth and Judiesch, 2007). Updates on latest SSR changes observed since 2000 provide a less coherent picture (Wild, 2012). They suggest a continuation of brightening at sites in Europe, USA, and parts of Asia, a levelling off at sites in Japan and Antarctica, and indications for a renewed dimming in parts of China (Wild et al., 2009; Xia, 2010a). The longest observational SSR records, extending back to the 1920s and 1930s at a few sites in Europe, further indicate some brightening during the first half of the 20th century, known as ‘early brightening’ (cf. Figure 2.13) (Ohmura, 2009; Wild, 2009). This suggests that the decline in SSR, at least in Europe, was confined to a period between the 1950s and 1980s. A number of issues remain, such as the quality and representativeness of some of the SSR data as well as the large-scale significance of the phenomenon (Wild, 2012). The historic radiation records are of variable quality and rigorous quality control is necessary to avoid spurious trends (Dutton et al., 2006; Shi et al., 2008; Gilgen et al., 2009; Tang et al., 2011; Wang et al., 2012e; Sanchez-Lorenzo et al., 2013). Since the mid-1990s, high-quality data are becoming increasingly available from new sites of the Baseline Surface Radiation Network (BSRN) and Atmospheric Radiation Measurement (ARM) Program, which allow the determination of SSR variations with unprecedented accuracy (Ohmura et al., 1998). Alpert et al. (2005) and Alpert and Kishcha (2008) argued that the observed SSR decline between 1960 and 1990 was larger in densely populated than in rural areas. The magnitude of this ‘urbanization effect’ in the radiation data is not yet well quantified. Dimming and brightening is, however, also notable at remote and rural sites (Dutton et al., 2006; Karnieli et al., 2009; Liley, 2009; Russak, 2009; Wild, 2009; Wang et al., 2012d). Globally complete satellite estimates have been available since the early 1980s (Hatzianastassiou et al., 2005; Pinker et al., 2005; Hinkelman et al., 2009). Because satellites do not directly measure the surface fluxes, they have to be inferred from measurable TOA signals using empirical or physical models to remove atmospheric perturbations. Available satellite-derived products qualitatively agree on a brightening from the mid-1980s to 2000 averaged globally as well as over oceans, on the order of 2 to 3 W m–2 per decade (Hatzianastassiou et al., 2005; Pinker et al., 2005; Hinkelman et al., 2009). Averaged over land, however, trends are positive or negative depending on the respective satellite product (Wild, 2009). Knowledge of the decadal variation of aerosol burdens and optical properties, required in satellite retrievals of SSR and considered relevant for dimming/brightening particularly over land, is very limited (Section 2.2.3). Extensions of satellite-derived SSR beyond 2000 indicate tendencies towards a renewed dimming at the beginning of the new millennium (Hinkelman et al., 2009; Hatzianastassiou et al., 2012). Reconstructions of SSR changes from more widely measured meteorological variables can help to increase their spatial and temporal coverage. Multi-decadal SSR changes have been related to observed changes in sunshine duration, atmospheric visibility, diurnal temperature range (DTR; Section 2.4.1.2) and pan evaporation (Section 2.5.3). Overall, these proxies provide independent evidence for the existence of large-scale multi-decadal variations in SSR. Specifically, widespread observations of declines in pan evaporation from the 1950s to the1980s were related to SSR dimming amongst other factors (Roderick and Farquhar, 2002). The observed decline in DTR over global land surfaces from the 1950s to the 1980s (Section 2.4.1.2), and its stabilisation thereafter fits to a large-scale dimming and subsequent brightening, respectively (Wild et al., 2007). Widespread brightening after 1980 is further supported by reconstructions from sunshine duration records (Wang et al., 2012e). Over Europe, SSR dimming and subsequent brightening is consistent with concurrent declines and increases in sunshine duration (Sanchez-Lorenzo et al., 2008), evaporation in energy limited environments (Teuling et al., 2009), visibility records (Vautard et al., 2009; Wang et al., 2009b) and DTR (Makowski et al., 2009). The early brightening in the 1930s and 1940s seen in a few European SSR records is in line with corresponding changes in sunshine duration and DTR (Sanchez-Lorenzo et al., 2008; Wild, 2009; Sanchez-Lorenzo and Wild, 2012). In China, the levelling off in SSR in the 1990s after decades of decline coincides with similar tendencies in the pan evaporation records, sunshine duration and DTR (Liu et al., 2004a; Liu et al., 2004b; Qian et al., 2006; Ding et al., 2007; Wang et al., 2012d). Dimming up to the 1980s and subsequent brightening is also indicated in a set of 237 sunshine duration records in South America (Raichijk, 2011).